In casting turbine blades using both single crystal and polycrystalline directional solidification investment casting techniques, ceramic cores are used. To form a turbine blade having the intricate internal passageways required for proper air cooling of the blade, a superalloy is cast around a ceramic core. After the casting has solidified, the ceramic core is chemically leached out of the casting using hot aqueous caustic solutions thus leaving the air cooling passageways in the blade.
The ceramic core used in the casting of superalloys to form turbine blades must be chemically inert with any reactive alloy components of the particular superalloy being cast. The ceramic core must also be capable of being leached out of the casting in a reasonable period of time without damaging the casting. In addition, the ceramic core must possess sufficient dimensional stability to maintain its shape when surrounded by the molten superalloy during solidification. However, the strength of the core must not be so high that hot tearing or recrystallization of the casting occurs during solidification and cooling of the casting.
Other preferable characteristics of the ceramic core include low shrinkage during sintering and a degree of porosity sufficient to provide for crushability of the core during solidification of the casting to reduce the potential for hot tearing and/or recrystallization of the superalloy.
To achieve higher efficiency in a gas turbine engine, it is well known that the operating temperature of the engine must be increased. However, the increase in gas turbine engine operating temperatures is limited by the availability of high temperature materials capable of withstanding these increased operating temperatures.
Recently, a new generation of high melting point superalloys, many of which are reactive with conventional fused silica cores, has been developed. These superalloys are typically cast at temperatures in the range from about 1480.degree. C. to about 1600.degree. C. While these superalloys allow higher operating temperatures and, hence, higher efficiencies to be achieved in gas turbine engines, they present problems in that conventional silica-based ceramic cores cannot withstand the higher temperatures at which these materials are cast.
In particular, typical conventional fused silica, alumina (Al.sub.2 O.sub.3), and alumina-based cores do not remain dimensionally stable at casting temperatures in excess of about 1480.degree. C. To meet the strict tolerance requirements of the aerospace industry for cast components, the core must remain dimensionally stable at such casting temperatures. Thus, it is apparent that conventional fused silica and alumina-based cores are not suitable for the casting of aerospace components at temperatures in excess of about 1480.degree. C.
Conventional yttria (Y.sub.2 O.sub.3) cores remain dimensionally stable at higher casting temperatures. However, yttria is extremely expensive and, consequently, the use of yttria cores in large scale production is uneconomical. Moreover, yttria cores have relatively poor leachability using standard caustic solutions.
Core materials formed of mixtures of yttria and alumina are also known in the art. However, single phase core materials such as 3Y.sub.2 O.sub.3.5Al.sub.2 O.sub.3 (yttria alumina garnet or YAG), presumably due to the presence of relatively large amounts of yttria, have relatively poor leachability using standard caustic solutions.
Consequently, there is a demand for a ceramic core that has excellent leachability and remains dimensionally stable at casting temperatures in excess of 1480.degree. C. It would also be desirable to form such a ceramic core principally form relatively inexpensive raw materials so that it is economical to use cores formed of the material in large scale production.
Accordingly, it is an object of the invention to provide a ceramic core for use in casting that has excellent leachability and remains dimensionally stable at casting temperatures in excess of 1480.degree. C.
Another object of the invention is to provide a ceramic core that is principally formed from relatively inexpensive raw materials so that it is economical to use cores formed of the material in large scale production.
It is a further objective of the invention to provide a ceramic core that remains chemically inert with the reactive alloy components of high melting point superalloys during the casting process.
A still further object of the invention is to provide a ceramic core that can be rapidly sintered and that exhibits minimal shrinkage during sintering.
Additional objects and advantages will be set forth in part in the description which follows, and in part, will be obvious from the description, or may be learned by practice of the invention.